1. Field of the Invention
The invention relates to a fiber reinforced hose. More specifically, the invention relates to improvements of durability of fiber reinforced hoses used for high temperature applications.
2. Description of the Related Art
Fiber reinforced hoses with a fiber reinforced layer are conventionally used. In case of a fiber reinforced hose for the circulatory system of liquid line with a low to medium operating pressure of 30 kPa to 2 MPa (maximum pressure: 150 kPa to 4 MPa) and operating temperature of 120° C. or less (maximum 150° C.), reinforcing fibers made of polyethylene terephthalate (PET) and aliphatic polyamide (nylon) are normally used.
In accordance with the increase of environmental temperatures inside the automobile engine compartment, demands for the thermal resistance of hoses are ever increasing. As a result, more materials with better heat resistance have come to be used as the rubber materials for hose, while reinforcing fibers made of aromatic polyamide with excellent heat resistance (aramid fibers) are often used.
For example, JP 3077169 and JP-A-2000-18444 disclose composite hoses or media transporting hoses having fiber reinforced layers using fibers made of PET, nylon, aramid or Kevlar™ (a trade name of aromatic polyamide) fibers.
It is well known that there are many types of reinforcing fibers. A fiber consisting of a combination of multiple monofilaments that are twisted is called a filament fiber. A filament fiber can be exemplified by a combination of 1000–2000 long single fibers of 0.5–2.0 denier twisted together. A spanized yarn is produced by bundling many monofilaments, stretching-breaking the monofilaments to a length of approximately 60 cm at an arbitrary position of each monofilament by applying a tensile force with a certain processing mechanism, and twisting thus obtained monofilaments together. A staple yarn is produced by twisting a group of fibers pulled out of a mass of monofilaments. Both spanized yarns and staple yarns are fibers with much fluff.
As can be easily seen from the above characteristics, the filament yarn generally has a large tensile strength. This means that it provides a high pressure resistance when it is used in a fiber reinforced layer of a hose. However, since the filament yarn has no fluff, it causes insufficient adherence with the rubber layers when it is used as a fiber reinforced layer. On the contrary, a fiber reinforced layer of spanized yarns or staple yarns with much fluff provides an excellent anchoring effect against the adjacent rubber layers, resulting in a good adhesiveness to the rubber layers. However, in order to use staple yarns as the reinforcing fibers of a hose, it is necessary to use yarns of a higher denier, i.e., to make the yarns thicker, in order to obtain a high pressure resistance. Therefore, the use of the staple yarn is not preferable due to a cost increase. The spanized yarn is suitable as the reinforcing fiber for the fiber reinforced hose, because it provides a good tensile strength as well, as can be seen from the production process described above.